1. Field of the Invention
The present invention relates to an SMIF system, more particularly to an apparatus and a method for preventing a wafer mapping system of an SMIF system from being polluted by corrosive gas remaining on wafers.
2. Description of the Related Art
It is well-known in semiconductor industry that an SMIF system is used to reduce particle fluxes onto semiconductor wafers during storage and transportation of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transportation, the gaseous media (such as air or nitrogen) surrounding the wafers is -essentially a stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter in the immediate wafer environment.
FIG. 1 shows a schematic front view of a conventional SMIF system 10 in which reference numeral 12 designates a cassette, 14 designates wafers stored in the cassette 12, and 16 designates a wafer mapping system. During fabrication, semiconductor wafers may undergo more than 300 process steps before the wafer is cut into individual integrated circuit chips, and the wafers must be transferred to and from an SMIF pod in each of these process steps. Each time a group of wafers are removed from and returned to a pod for the individual process, there is a danger that one or more of the wafers will be damaged or destroyed because the wafers are improperly positioned. Silicon semiconductor wafers are extremely valuable, with each wafer costing up to one-thousand US dollars. More significantly, as devices are formed on the wafer surfaces, the worth of a particular lot of wafers within a pad may exceed one-million dollars. It is therefore critical to avoid damage or loss to the wafers during fabrication.
In order to prevent damage due to improper position of wafers and in order to avoid a waste of time accessing dummy wafers, the wafer mapping system 16 is used to detect the presence and position of wafers in wafer slots. This information may then be stored, as in a computer memory, to provide a wafer map for a particular pod or SMIF pod for later use. Referring to FIG. 2, a top view of the wafer mapping system 16 is shown. The wafer mapping system 16 includes sensors 17 and mirrors 18 and a laser 19. FIG. 3 shows a schematic enlarged view of a part of the wafer mapping system 16 including the mirrors 18 and laser 19. The laser 19 functions as a transmitter while the sensors functions as a receiver. Moreover, the mirrors 18 are used to reflect and/or split a light 13 emitted from the laser 19. During the operation, the sensors 17 receive lights 15 coming from the mirrors 18 with the wafers 14 therebetween, thereby detecting the presence and positions of the wafers 14.
However, the mirrors 18 may be polluted by corrosive gases remaining on the wafers 14, causing changes to the refractive indexes of the mirrors 18, even damages to the mirrors 18. Under this condition, the lights coming from the mirrors 18 will deviate from the original paths. As a result, the presence and positions of the wafers cannot be precisely detected.
In view of the above, an object of the present invention is to provide an apparatus for preventing a wafer mapping system of an SMIF system from being polluted by a corrosive gas remaining on wafers. The wafer mapping system includes a plurality of mirrors and sensors used to detect the positions of the wafers. The apparatus of the present invention includes a pipe having a plurality of holes thereon and a purge gas flowing inside the pipe, and is characterized in that the purge gas is emitted out from the plurality of holes toward the mirrors of the wafer mapping system, thereby preventing the mirrors from being polluted by the corrosive gas remaining on the wafers.
The apparatus of the present invention further includes a plurality of valves for controlling the amount of the purge gas flowing inside the pipe and an outlet for allowing the emitted purge gas to be exhausted out of the SMIF system.
Another object of the present invention is to provide a method for preventing a water mapping system of an SMIF system from being polluted by a corrosive gas remaining on wafers. The water mapping system includes a plurality to mirrors and sensors used to detect the positions of wafers. The method of the present invention is characterized by emitting a purge gas from a pipe toward the mirrors of the wafer mapping system, thereby preventing the mirrors from being polluted by the corrosive gas remaining on the wafers.
In the method of the prevent invention, the amount of the purge gas emitted is controllable by a plurality of valves, and the emitted purge gas can be exhausted out of the SMIF system through an outlet.